1. Field of the invention
The present invention relates to a current-injection type quantum wire laser utilizing a line shape density in order to obtain a semiconductor laser with characteristics such as a small driving current value, less temperature dependency, and a narrow spectral line width.
2. Description of the prior art
In the past, a quantum well laser has been developed having a density of state in a stair shape due to a quantum well having a two-dimensional quantum wire. Moreover, a quantum wire laser has been developed as a quantum effect laser having a density of state in a line spectrum shape due to a one-dimensional quantum wire.
FIGS. 7a and 7b represent a method for producing a quantum wire laser contrived in the past (See Japanese Laid-Open Patent Publication No. 64-21986). FIGS. 7a and 7b are a sectional view showing a quantum wire laser and a sectional shape of a ridge or a growth layer on the ridge, respectively. The reference numeral 71 denotes an n-GaAs substrate, 72 an n-AlGaAs cladding layer, 73 an n-AlGaAs graded layer, 74 a GaAs quantum well layer, 75 a p-AlGaAs graded layer, 76 a current blocking layer, 77 a p-AlGaAs cladding layer, and 78 a p-GaAs contact layer. The sectional shape shown in FIG. 7b can be produced by forming a ridge on the n-AlGaAs cladding layer 72 in a [110] direction and successively growing the n-AlGaAs graded layer 73, the GaAs quantum well layer 74, the p-AlGaAs graded layer 75, the current blocking layer 76, the p-AlGaAs cladding layer 77, and the p-GaAs contact layer 78 by Metal Organic Chemical Vapor Deposition (MOCVD). The growth of this structure utilizes the characteristics of the MOCVD method that the growth rate on a (111) B facet (slant faces of a triangle in FIG. 7) is markedly smaller than that on a (100) surface. The GaAs quantum well layer 74 is surrounded by the n-AlGaAs graded layer 73, the current blocking layer 76, and the p-AlGaAs graded layer 75. A width W.sub.2 of the GaAs quantum well layer 74 on the side of the graded layer 73, a width W.sub.1 of the GaAs quantum well layer 74 on the side of the graded layer 75, and a thickness b of the GaAs quantum well layer 74 are formed so as to be 194 .ANG., 123 .ANG., and 50 .ANG., respectively. These values are as small or smaller than the de Broglie wavelength, so that a quantum wire is formed in the GaAs quantum well layer 74.
However, there is a problem in the quantum wire laser shown in FIGS. 7a and 7b. More particularly the quantum wire typically has insufficient geometrical precision. In order for the quantum wire to satisfactorily function, it is required that the absolute value of the quantized level of the quantum wire be set with a precision on the order of angstroms. It will be appreciated that the accuracy of the thickness b of the GaAs quantum well layer 74 is determined by the growth rate on the (100) facet. The growth rate of a semiconductor can be regulated with a sufficient precision on the order of angstroms by high performance growth methods such as the MOCVD method. The widths of W.sub.1 and W.sub.2 of the GaAs quantum well layer 74 are determined by a ridge width l of the n-AlGaAs cladding layer 72 and a height h.sub.1 of the n-AlGaAs graded layer 73. The accuracy of the ridge width l depends on the accuracy provided by the light or electron beam lithography. The precision of the lithography is 0.1 pm, so that the precisions of the widths W.sub.1 and W.sub.2 are 0.1 .mu.m. Thus, the absolute value of the quantization level of the quantum wire cannot be regulated with good reproducibility and because of the above-mentioned problem, it has been difficult in the past to produce a quantum wire laser having a line shape density of state.